Cell growth is regulated by signal transduction pathways that respond to extracellular stimuli. In yeast, the signal that promotes growth is thought to be the availability of nutrients. Nutritional signaling involves the activation of Ras proteins and adenylyl cyclase through a process that is not well understood. Results presented in this proposal demonstrate that nutritional signaling in yeast also involves the activation of a G protein alpha-subunit by a G protein-coupled receptor. Several observations suggest that this pathway is activated in response to nitrogen starvation. For example, strains lacking the receptor or the G protein are defective for pseudohyphal growth, a developmental switch that is induced by nitrogen starvation. The long-term objective of this project is a description of the mechanisms employed by the G protein-mediated pathway to signal in response to nutritional conditions. Classical genetics and protein biochemistry will be used to elucidate the roles of genes and their encoded proteins in this process. The aims of this project are to characterize and identify the ligand for a newly described G protein-coupled receptor and to determine when the receptor is physiologically active. The first specific aim will investigate the conditions under which the Gpr1p receptor is bound to its ligand. These studies will be performed by coupling the Gpr1p receptor to the pheromone response pathway and using a well-established reporter construct that detects activation of the pathway. The second specific aim will employ the Gpr1p reporter system to characterize and identify the Gpr1p ligand. Identification of such a ligand would be one of the first examples of a receptor-binding molecule that activates a nutritional signaling pathway in a eukaryotic organism. The final specific aim will characterize the regulation of Gpr1p receptor expression and stability. Investigation of this novel signaling system will contribute to an understanding of nutritional signaling in eukaryotes, an area in which many important questions have not yet been addressed. Significant progress in elucidating signal transduction mechanisms has been made in organisms in which sophisticated genetic manipulations can easily be performed. A complete understanding of growth control in yeast is likely to provide insight into growth control in mammals, which is also regulated by signaling through G proteins and Ras. Such information is essential for understanding the process of carcinogenesis, in which cells escape this regulation and exhibit uncontrolled growth.